Light-emitting diodes produced from a nitride compound obtained with one or more materials in particular including gallium (GaN) and capable of emitting in the blue, have been well known for already more than a decade. However, their aptitude to also be able to effectively emit at a range of longer wavelengths, typically in the range of the visible spectrum corresponding to the green and to the red, runs up against intrinsic difficulties related to the crystal symmetry of the materials used, which limits the current-light conversion efficiency of diodes designed to emit in this wavelength range. Specifically, to be able to emit wavelengths longer than the blue requires recourse to be made, to form the active light-emitting zone, to nitride alloys of gallium and indium (GaInN) including high concentrations of indium. A problem that arises is that of how to incorporate the increasing proportions of indium that are then necessary into the GaInN alloys without degrading the structural quality of the emitting zones and therefore the emission capacity thereof.
A second problem that arises for the entire range of wavelengths that may be produced with these nitrides, blue included, is related to the crystal symmetry of this family of materials. Specifically, these materials of hexagonal symmetry, if they are grown epitaxially in a principal crystal direction called the “c” direction, develop a spontaneous and piezoelectric polarization along this direction. An internal electric field is created that has the negative effect of separating electrons and holes spatially in the active emission zone, this translating directly into a loss of radiative efficiency. Since the polarization is directed along the c axis, the “polar” orientation of the crystal, it is advantageous to use epitaxial growth directions that are inclined with respect to this axis, for which directions the polarization component will be lower or even zero, as illustrated in FIG. 1. FIG. 1 also shows the influence of the proportion of indium 130 contained in the alloy on this parameter. These directions are commonly called “nonpolar” directions 110 or “semipolar” directions 120. Moreover, the incorporation of indium into the nitrides employed may be facilitated when the epitaxial growth is carried out from surfaces corresponding to certain of these orientations. It will therefore be understood how important it is to privilege such crystal orientations in order to increase the performance of green LEDs and, above all, LEDs emitting at a range of longer wavelengths extending from the yellow to the red, the efficiency of which is at the present time still too low for potential industrial use thereof to be envisionable.
Known methods for obtaining epitaxially grown layers of nitride compounds such as those mentioned above, in nonpolar and semipolar directions, are briefly described below.
The problem to be addressed is that of determining which substrate to use to allow an epitaxial growth in nonpolar directions 110 or semipolar directions 120. If it is desired to simultaneously minimize the defect concentration in the epitaxially grown layers, the most suitable method is to use substrates of the same nature as the layers to be epitaxially grown (homo-substrates). In the case of the aforementioned nitrides, bulk ingots of GaN, drawn in the c direction, are still only of small size, typically of a diameter smaller than 1 inch (2.5 cm), this meaning that it is not possible to cut therefrom substrates that are inclined with respect to the c axis of sufficient size for the envisioned industrial applications, these substrates then typically being smaller than a few cm2 in size.
One solution that is not affected by the above dimensional problem consists in using layers deposited in the right crystal direction on a substrate of suitable orientation, for example a sapphire substrate of larger size and of suitable orientation. These layers, which are commonly called templates, acquire the desired orientation, i.e. a nonpolar orientation 110 or semipolar orientation 120, via quasi “heteroepitaxial” growth on the chosen substrate, which is for example made of sapphire. However, the layers thus obtained are observed to be riddled with a high number of stacking defects that extend in the c plane, inclined with respect to the surface, and that therefore emerge on the surface of the grown layer and, to a lesser extent, with a certain number of dislocations. The epitaxial growth of these templates merely increases the length of these defects. When these defects cross the active zones, they induce therein non-radiative recombination or radiative recombination at shorter wavelengths. This at least partially explains the lower current-light conversion efficiency of LEDs manufactured from such layers.
In order to attempt to mitigate these difficulties, recourse may be had to what are called “epitaxial lateral overgrowth” (ELO) methods. At a certain stage in the growth of the layer, a mask 201 is deposited with the aim, on the one hand, of blocking dislocations under the mask and, on the other hand, of causing the remaining dislocations to curve during the lateral overgrowth that occurs over the mask. Such a method is for example described in the following publication, which was published in Semiconductor Science and Technology Volume 27 Number 2 (2012), entitled “Defect reduction methods for III-nitride heteroepitaxial films grown along nonpolar and semipolar orientations” by P. Vennéguès and co-authors. As shown in FIG. 2, although defects and dislocations under the mask 201 are observed to indeed be effectively blocked, the stacking defects do not curve like the dislocations and those 203 that manage to propagate through the apertures 202 of the mask may reach the surface. The right-hand illustration of FIG. 2 is a schematic view of the left-hand illustration, which is a photo.
Other solutions have therefore been developed that attempt, rather than to block the stacking defects, to avoid creating them. This type of method is based on “faceting” substrates so as to initiate the growth locally on facets created on the surface of said substrates, which facets allow GaN to be grown epitaxially in the c, i.e. (0001) direction, as shown in FIGS. 3a to 3f. As shown in FIG. 3b, beyond the facets 330, the substrate is covered with a dielectric mask 320. In this case, stacking defects, generated at the start of growth, and which by nature are aligned with the c {0001} plane are confined in a thin zone close to the interface between the facet and the layer. In addition, the growth of the crystallites on the facets is accompanied by curvature of the dislocations in the first moments of growth. The originality of this approach resides in the fact that the orientation of the substrate 300, and therefore the inclination of the facets, is chosen so that the coalescence of the various crystallites produces, in the end, as shown in FIG. 3d, a planar and continuous GaN surface 350 with the desired semipolar orientation. The zone containing stacking defects is very small in size, typically of a few nanometers in thickness. Such methods have been developed by various laboratories on substrates made of silicon or of sapphire. The reader may for example refer to the following publications: “T. Honda et al., Journal of Crystal Growth 242 (1-2), 82 (2002)”; “B. Leung et al., Applied Physics Letters 104 (26) (2014)” and “T. Tanikawa et al., Physica Status Solidi (C) 5 (9), 2966 (2008)”. For substrates made of sapphire and substrates made of silicon, the facets 330 are revealed chemically or by dry etching. FIGS. 3a to 3d illustrate the succession of steps required to obtain the continuous layer 350 starting, for example, with a {001} silicon substrate 300 with a disorientation of 7°. For growth on silicon, facets 330 of {111} orientation are revealed by chemical etching using KOH or potassium hydroxide. The starting substrate is masked and the chemical etching takes place through apertures 370 in the mask, thus forming grooves 360. The etch time sets the etching depth 380 and therefore the height of the {111} facets 330 exposed. As described above, and since the GaN growth has a +c orientation on the {111} facets 330 of the silicon, the initial orientation of the silicon is precisely chosen so as to select the desired semipolar orientation of the surface of the GaN layer 350. This has allowed on the whole satisfactory results to be obtained on various silicon orientations and therefore for various GaN-layer semipolar orientations. An experimental example is illustrated in FIGS. 3e and 3f in the case of growth of GaN on {001} silicon with a disorientation of 7° in the <110> direction. It will be noted here that the use of silicon substrates is always preferable insofar as silicon is the material that is most commonly used in the microelectronics industry and thus large silicon substrates are available at low cost.
Although providing certain improvements, the methods of localized heteroepitaxial semipolar growth which were briefly described above still have many limitations.
In particular, growth of GaN on a silicon substrate runs up against additional difficulties related to the appearance of an effect called “melt-back etching” in the step of growth of the nitride layer such as a layer of GaN. This destructive effect is a result of the reactivity of silicon with gallium. Specifically, in the phase of growth of the crystallites, the silicon sees its temperature increase enough that it can react with the gallium. This reaction generally leads to the creation of cavities in the silicon.
These cavities decrease the quality of the substrate and therefore LED performance. Moreover, they appear randomly on the surface of the silicon, this resulting in LEDs obtained from a given stack of layers having a low uniformity.
In order to avoid this untimely etching of the silicon by the gallium, it is possible to deposit a buffer layer of aluminum nitride (AlN) on the silicon before starting the GaN growth. Although this buffer layer of aluminum nitride (AlN) allows the “melt-back etching” effect to be limited, in practice it is rare to be able to completely eliminate it.
There is therefore a need to provide a solution allowing a nitride layer, for example a layer of gallium nitride, of semipolar orientation, to be obtained from facets oriented in the {111} crystal plane of a silicon layer and that allows the appearance of the melt-back-etching effect to be further decreased.
Other aims, features and advantages of the present invention will become apparent on examining the following description and the accompanying drawings. It will be understood that other advantages may be incorporated.